1. Field of the Invention
The present invention relates generally to an exhaust system for an automotive vehicle and more specifically to a exhaust system which suppress exhaust noise.
2. Description of the Prior Art
FIG. 1 of the drawings shows a previously proposed arrangement disclosed in Utility Model Provisional Publication No. 58-134620. This arrangement includes an exhaust manifold 100 for a four cylinder in-line internal combustion engine and which is formed in a manner that branch runners 101, and 105 communicate with exhaust ports of cylinders #1 and #4 while the remaining two branch runners 102 and 103 communicate with ports of cylinders, #2 and #3. As shown, branch runners 101 & 104 and 102 & 103 are merged to form collector sections 105 and 106 respectively and thus define two isolated or discrete exhaust passage structures.
The collector sections 105, 106 communicate with a front tube arrangement 120 which in this instance is formed of first and second discrete conduits 122 and 124. The manifold 100 and the front tube 120 arrangement are provided with flanges 107 and 125 which are detachably interconnected by bolts. The downstream end of the front tube arrangement is provided with a collector-like junction member 126 which fluidly interconnects the downstream ends of conduits 122 and 124. A pre-muffler 130 is fluidly communicated at its upstream end with the junction member 126 via a first connection conduit 132 and at its downstream end with a main muffler 140 via a second connection conduit 134 and a center tube 142. A tail tube 144 leads from the main muffler to the ambient atmosphere.
The firing order of the engine is arranged to be #1 - #3 - #4 - #2. Accordingly, with the above arrangement exhaust gas slugs or pulses are supplied alternatively into the above mentioned separate passage structures of the manifold in a manner which promotes filling efficiency and reduces backpressure.
However, this arrangement has suffered from the drawback that under certain conditions such as during rapid acceleration or during a transition between high engine speed and low engine speed a metallic exhaust noise containing a high frequency component is produced.
Initially this noise was thought to be caused by combustion in the exhaust system of residual fuel contained in the exhaust gases. However, measurement revealed that during deceleration with the throttle valve closed and the engine motoring there was insufficient fuel in the exhaust gases to enable the generation of the noise in question. It was then determined that the cause of the noise was due to the formation of a standing wave in the exhaust conduiting which is converted into a shockwave as it moves through the main muffler and tail tube.
More specifically, the mechanism via which the noise is generated is deemed to be such that as exhaust gases are released into the exaust manifold (for example into branch runner 101 from cylinder #1) the pressure wave is trasmitted via collector section 105 and conduit 122 to the junction member 126. Upon reaching this point in the system the wave is transmitted to the pre-muffler 130 via conduit 132 and also back toward the engine via conduit 124 and collector section 106. However, as the exhaust valves of the cylinders associated with branch runners 102 and 103 are still closed at this time the pressure wave which has run back toward the engine tends to be be reflected to form a reflected wave which then propagates back toward the junction member 126. Subsequently, the next slug of exhaust gas is released from the engine (for example from cylinder #3 into branch runner 103) and follows the reflected wave. Under given operational conditions these waves tend to cooperate in a manner which produces a kind of resonance and establishes a large amplitude standing wave. This standing wave is converted into a shock wave as its moves through the front tube 120, center tube 142, main muffler 140 and tail tube 144 and generates an audible metallic exhaust noise which includes a high frequency component.